Exposure apparatus and device manufacturing method

ABSTRACT

An exposure apparatus includes a projection optical system ( 3 ) for projecting a pattern of a mask ( 2 ) onto a substrate ( 5 ), and a fluid supply unit ( 6 ) for supplying a fluid between said projection optical system and the substrate, said fluid supply unit ( 6 ) including an injection unit ( 19 ) for injecting carbon dioxide into the fluid.

TECHNICAL FIELD

This invention relates generally to an exposure apparatus that utilizesan immersion method, and is suitable, for example, for the lithographyprocess for manufacturing highly integrated devices, such assemiconductor devices, e.g., ICs and LSIs, image pick-up devices, e.g.,CCDs, display devices, e.g., a liquid crystal panels, communicationdevices, e.g., optical waveguides, and magnetic heads by transferring apattern of a mask (or a reticle) onto a photosensitive agent appliedsubstrate.

BACKGROUND ART

An exposure apparatus for exposing a mask pattern onto aphotosensitive-agent applied substrate have conventionally been used tomanufacture semiconductor devices and liquid crystal panels. Since finerprocessing of a pattern is demanded for improved integrations ofdevices, exposure apparatuses are improved so as to resolve finepatterns.

The following Rayleigh equation (1) defines resolution R of a projectionoptical system in an exposure apparatus, which is an index of a size ofa resolvable pattern:R=k ₁(λ/NA)  (1)where λ is an exposure wavelength, NA is a numerical aperture of theprojection optical system at its image side, and k₁ is a constantdetermined by a development process and others, which usually isapproximately 0.5.

As understood from Equation (1), the resolving power of the opticalsystem in the exposure apparatus becomes higher as the exposurewavelength is shorter and the image-side NA of the projection opticalsystem is greater.

Therefore, following the mercury lamp i-line (with approximately 365 nmin wavelength), a KrF excimer laser (with approximately 248 nm inwavelength) and an ArF excimer laser (with approximately 193 nm inwavelength) have been developed, and more recently an F₂ excimer laser(with approximately 157 nm in wavelength) is reduced to practice.However, a selection of the exposure light having a shorter wavelengthmakes it difficult to meet material requirements with respect totransmittance, uniformity and durability, etc., causing an increasingcost of the apparatus.

An exposure apparatus having a projection optical system with a NA of0.85 is commercially available, and a projection optical system with aNA of 0.9 or greater is researched and developed. Such a high-NAexposure apparatus has difficulties in maintaining good imagingperformance with little aberration over a large area, and thus utilizesa scanning exposure system that synchronizes the mask with a substrateduring exposure.

However, a conventional design cannot make the NA greater than 1 inprinciple due to a gas layer having a refractive index of about 1between the projection optical system and the substrate.

On the other hand, an immersion method is proposed as means forimproving the resolving power by equivalently shortening the exposurewavelength. It is a method used for the projection exposure, which fillsliquid in a space between the final surface of the projection opticalsystem and the substrate, instead of filling this space with air as inthe prior art. The projection exposure apparatus uses, as the immersionmethod, a method for immersing the final surface of the projectionoptical system and the entire substrate in the liquid tank (see, forexample, Japanese Patent Application, Publication No. 6-124873), and alocal fill method that flows the fluid only in the space between theprojection optical system and the substrate (see, for example,International Publication No. WO99/49504 pamphlet).

The immersion method has an advantage in that the equivalent exposurewavelength has a wavelength of a light source times 1/n, where n is arefractive index of the used liquid. This means that the resolving powerenhances by 1/n times the conventional resolving power, even when thelight source having the same wavelength is used.

For example, when the light source has a wavelength of 193 nm and thefluid is the water, the refractive index is about 1.44. Therefore, useof the immersion method can improve the resolving power by 1/1.44 timesthe conventional method.

The most common fluid used for the immersion method is water. The waterhas a good transmittance relative to the ultraviolet light down to about190 nm. In addition, advantageously, a large amount of water is used inthe semiconductor manufacturing process, and the water gets along withthe wafer and photosensitive agent.

It is important to reduce the influence of the air gas bubbles to theexposure in the immersion exposure apparatus. These gas bubbles thatenter the exposure area between the final surface of the projectionoptical system and the substrate scatter the exposure light. Therefore,the transferred pattern's critical dimension varies beyond thepermissible range, causing insulations and short circuits contrary tothe design intent in the worst case. Degassing of the fluid is the mosteffective, known method to prevent the influence of the gas bubbles tothe exposure. Since the gas bubbles are unlikely to occur or thegenerated gas bubbles extinguish in a short time period in the degassedfluid, the influence of the gas bubbles to the exposure is prevented.

However, the water widely used for the immersion method increases theresistivity, when degassed, and is likely to generate static electricitydisadvantageously. For example, the pure water (i.e., the watercontaining few impurities) used for the semiconductor manufacturingprocess reaches the resistivity of 18 MΩ·cm after degas. The substratesurface is electrically insulated since the photosensitive agent isapplied on it. Therefore, as the stage moves the substrate, the staticelectricity is generated on the substrate surface and makes the deviceon the substrate defective.

DISCLOSURE OF INVENTION

With the foregoing in mind, the present invention has an exemplaryobject to provide an exposure apparatus that uses an immersion methodwhile reducing the static electricity on the substrate.

An exposure apparatus according to one aspect of the present inventionincludes a projection optical system for projecting a pattern of a maskonto a substrate, and a fluid supply unit for supplying a fluid betweenthe projection optical system and the substrate, the fluid supply unitincluding an injection unit for injecting carbon dioxide into the fluid.

An exposure apparatus according to another aspect of the presentinvention includes an illumination optical system for illuminating amask using light from a light source, and a projection optical systemfor projecting a pattern of the mask onto a substrate, wherein a fluidsupplied to a space between the projection optical system and thesubstrate has a concentration of carbon dioxide between 0.02 ppm and 750ppm.

An exposure apparatus according to still another aspect of the presentinvention includes an illumination optical system for illuminating amask using light from a light source, and a projection optical systemfor projecting a pattern of the mask onto a substrate, wherein a fluidsupplied to a space between the projection optical system and thesubstrate has a resistivity value between 0.02 MΩ·cm and 10 MΩ·cm.

A device manufacturing method according to another aspect of the presentinvention includes the steps of exposing an object using the aboveexposure apparatus, and developing the exposed object.

Other objects and further features of the present invention will becomereadily apparent from the following description of the embodiments withreference to accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

FIG. 1 is a schematic view of a principal part in an exposure apparatusof a first embodiment.

FIG. 2 is a schematic view showing a structural example of an injectionunit of carbon dioxide.

FIG. 3 is an explanatory view showing a relationship between a potentialon a wafer and a pure water's resistivity in a wafer's cleansing step.

FIG. 4 is an explanatory view showing a relationship between the purewater's resistivity and the concentration of carbon dioxide.

FIG. 5 is an explanatory view showing a relationship between anormalized life of a gas bubble and a normalized concentration of adissolved gas.

FIG. 6 is an explanatory view showing a relationship between a water'sPH value and a concentration of the carbon dioxide.

FIG. 7 is a schematic view showing a principal part of an exposureapparatus as a variation of the first embodiment.

FIG. 8 is a manufacture flow of a device.

FIG. 9 is a wafer process shown in FIG. 8.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will now be described indetail in accordance with the accompanying drawings.

First Embodiment

FIG. 1 is a schematic view of a principal part of an exposure accordingto a first embodiment. This embodiment applies the present invention toa scanning exposure apparatus.

In FIG. 1, 1 denotes an illumination optical system for illuminating areticle (or a mask) with light from a light source. The light source isan ArF excimer laser (with a wavelength of 193 nm), a KrF excimer laser(with a wavelength of 248 nm), and F₂ laser, and the illuminationoptical system 1 includes a known optical system etc. (not shown). 3denotes a refracting or catadioptric or another projection opticalsystem for projecting a circuit pattern of a reticle 2 illuminated bythe illumination optical system 1, onto a wafer 5 (substrate) as asecond object. 15 denotes a distance measuring laser interferometer formeasuring a two-dimensional position on a horizontal plane of each of areticle stage 12 and a wafer stage 13 via a reference mirror 14. A stagecontroller 17 controls positioning and synchronizations of the reticle 2and the wafer 5 based on this measurement value. The wafer stage 13serves to adjust a position in a longitudinal direction, a rotationalangle, and an inclination of a wafer so that the surface of the wafer 5matches the image surface of the projection optical system 3.

This embodiment uses the immersion method to shorten the equivalentexposure wavelength, and improve the exposure resolution. Therefore,this embodiment arranges a supply port 10 and a recovery port 11 aroundthe final surface of the projection optical system 3, supply the waterbetween the final surface of the projection optical system 3 and thewafer 5, and form a liquid film 4 there. An interval between the finalsurface of the projection optical system 3 and the wafer 5 is preferablysmall enough to stably form the liquid film 4, such as 0.5 mm. Thesupply port 10 is connected via a supply tube 8 to a fluid supply unit 6that supplies the water. The recovery port 11 is connected via arecovery tube 9 to a fluid recovery unit 7 that recovers the water. Thefluid supply unit 6 includes a degassing unit 18 and a carbon dioxideinjection unit 19 provided at the downstream of the degassing unit 18.The degassing unit 18 can have, for example, a well-known membranemodule (not shown) and a vacuum pump (not shown). An immersioncontroller 16 sends a control signal to the fluid supply unit 6 and thefluid recovery unit 7, and transmits and receives data with a stagecontroller 17. Thereby, the immersion controller 16 adjusts the liquidsupply amount and recovery amount according to the wafer's movingdirection and speed.

This embodiment injects a predetermined concentration of carbon dioxideinto the degassed water, prevents influence of gas bubbles to theexposure, and restrains the static electricity on the substrate. Carbondioxide advantageously is inexpensive and does not contaminate thesubstrate. Referring to FIG. 2, a description will be given of oneexemplary structure of the carbon dioxide injection unit 19. A membranemodule 22 is provided between an inflow port 20 of the water and anoutflow port 21. The membrane module 22 is connected to a supply source24, such as a CO₂ cylinder, via a valve 23. The valve 23 is electricallycontrolled by a carbon dioxide controller 25.

This configuration restrains a concentration of the carbon dioxide inthe water by changing the flow of the carbon dioxide to the membranemodule 22 via the valve 23. A resistivity meter 26 is provided at adownstream side of the carbon dioxide injection unit. It is morepreferable to control the concentration of the carbon dioxide within apredetermined range by electrically feeding back an output of theresistivity meter 26 to the carbon dioxide controller 25. Carbon dioxidegas may be injected into the water from a nozzle instead of using amembrane module. In this case, it is preferable to eliminate fineparticles in the carbon dioxide gas using a filter in advance.

A description will now be given of an optimal concentration of carbondioxide.

The lower limit of the carbon-dioxide concentration is determined fromthe necessity to restrain the static electricity on the wafer. FIG. 3shows a relationship between the potential on the wafer and the water'sresistivity when the wafer is cleansed with pure water from the nozzle(as detailed in Asano, “Spray, Contact, Flow Charges of Pure Water andUltra-pure Water”, The Institute of Electrical Engineering of Japan,Vol. 108 (1988), pp. 362-366). It is understood from FIG. 3 that whenthe water's resistivity exceeds 10 MΩ·cm, the large potential is likelyto generate on the wafer. On the other hand, when the water'sresistivity is equal to or smaller than 5 MΩ·cm, the static electricitydoes not pose a problem. FIG. 4 shows a relationship between thecarbon-dioxide concentration and the resistivity in the pure water. Asthe carbon-dioxide concentration increases, the resistivity decreases.The carbon-dioxide concentrations of 0.02 ppm and 0.06 ppm correspond tothe resistivity of 10 MΩ·cm and 5 MΩ·cm. This means that thecarbon-dioxide concentration in the water is preferably 0.02 ppm orgreater, more preferably 0.06 ppm or greater, in order to restrain thestatic electricity on the wafer.

The upper limit of the carbon-dioxide concentration can be determinedfrom a problem of the gas bubbles. The gas-bubble generation is causedby a pressure variance in the water and inclusions of fine gas bubblesin the carbon-dioxide injection unit. In either case, as thecarbon-dioxide concentration in the water increases, the gas bubble islikely to generate and its life (or a time period during which agenerated gas bubble extinguishes due to diffusion) becomes long. Inother words, the gas bubble is unlikely to extinguish and a danger ofthe influence of the gas bubbles to the exposure increases. FIG. 5 showsthe normalized life τ/τ₀ of a gas bubble as a function of C_(∞)/C_(s) asa normalized concentration of dissolved gas (as detailed in C. E.Brennen, “Cavitation and Bubble Dynamics,” Oxford University Press(1995), Chapter 2). Here, τ₀ is the life when C_(∞)=0.0 and Cs is asaturated concentration. When normalized concentration C_(∞)/C_(s) is0.2 or smaller, the life of a gas bubble is close to C_(∞)=0.0 and thegas bubble is likely to extinguish relatively immediately. On the otherhand, when normalized concentration C_(∞)/C_(s) becomes 0.5 or greater,the life of a gas bubble drastically increases and gas bubble isunlikely to extinguish. As a result of this, in order to prevent theinfluence of the gas bubble to the exposure, the concentration of gasdissolved in the water is preferably 50% or smaller of the saturatedconcentration, and more preferably 20% or smaller of the saturatedconcentration. The saturated concentration of carbon dioxide in thewater is about 1500 ppm in one air pressure. Therefore, in order toprevent the influence of the gas bubbles to the exposure, thecarbon-dioxide concentration may be preferably 750 ppm or smaller, morepreferably 300 ppm. These values are much greater than the lower limitof the carbon-dioxide concentration necessary to restrain the staticelectricity, and it is possible to reconcile the prevention of the gasbubble's influence with the restraint of the static electricity.

In summary, the carbon-dioxide concentration in the water supplied tothe liquid film 4 ranges preferably from 0.02 ppm to 750 ppm, and morepreferably from 0.06 ppm to 300 ppm. The equivalent condition is thatthe resistivity ranges preferably from 0.02 MΩ·cm to 10 MΩ·cm, and morepreferably from 0.04 MΩ·cm to 5 MΩ·cm. This configuration prevents theinfluence of the gas bubbles to the exposure and restrains the staticelectricity on the substrate.

This embodiment can implement an acid environment suitable for achemical amplification type of resist. The chemical amplification typeof resist is widely used as the highly sensitive resist optimal to thelithography that uses a KrF laser and an ArF laser for a light source.On the other hand, when alkali contaminations in the water, such asammonia, enter the resist surface in the chemical amplification type ofresist, a chemical reaction is restricted and a problem on the pattern,such as T-top, occurs. This embodiment decreases the PH value bydissolving carbon dioxide in the water, and restrains the influence ofalkali contaminations. FIG. 6 shows a relationship between the water'sPH value and the carbon-dioxide concentration.

Usually, the semiconductor factory has the pure water facility, and manyof the pure water facilities have a degassing function. Where thedegassed water is supplied from the pure water facility outside theexposure apparatus to the projection exposure apparatus, the projectionexposure apparatus can omit the degassing unit 18. The omission of thedegassing unit 18 would reduce the cost. FIG. 7 shows a variation of theexposure apparatus according to the instant embodiment that implementsthe above concept. The exposure apparatus as a variation differs fromthat shown in FIG. 1 in that the fluid supply unit 6 has no degassingunit. A structure of the carbon-dioxide injection unit, the optimalcarbon-dioxide concentration in the water and resistivity in theexposure apparatus in this variation are the same as those in theexposure apparatus shown in FIG. 1.

Thus, the immersion exposure apparatus of this embodiment prevents theinfluence of the gas bubbles to the exposure and restrains the staticelectricity on the substrate.

Second Embodiment

A description will now be given of an embodiment of a devicemanufacturing method using the exposure apparatus of the firstembodiment.

FIG. 8 is a flowchart for explaining a fabrication of devices (i.e.,semiconductor chips such as IC and LSI, liquid crystal panels, andCCDs). Step 1 (circuit design) designs a device circuit. Step 2 (maskfabrication) forms a reticle having the designed circuit pattern. Step 3(wafer making) manufactures a wafer using materials such as silicon.Step 4 (wafer process), which is referred to as a preprocess, formsactual circuitry on the wafer through photolithography using the maskand wafer. Step 5 (assembly), which is also referred to as apostprocess, forms into a semiconductor chip the wafer formed in Step 4and includes an assembly step (e.g., dicing, bonding), a packaging step(chip sealing), and the like. Step 6 (inspection) performs various testsfor the semiconductor device made in Step 5, such as a validity test anda durability test. Through these steps, a semiconductor device isfinished and shipped (Step 7).

FIG. 9 is a detailed flow of the wafer process. Step 11 (oxidation)oxidizes the wafer's surface. Step 12 (CVD) forms an insulating film onthe wafer's surface. Step 13 (electrode formation) forms electrodes onthe wafer by vapor disposition and the like. Step 14 (ion implantation)implants ions into the wafer. Step 15 (resist process) applies aphotosensitive material onto the wafer. Step 16 (exposure) uses theexposure apparatus of the first embodiment to expose a circuit patternon the mask onto the wafer. Step 17 (development) develops the exposedwafer. Step 18 (etching) etches parts other than a developed resistimage. Step 19 (resist stripping) removes disused resist after etching.These steps are repeated, and multilayer circuit patterns are formed onthe wafer.

The device fabrication method of this embodiment may manufacture higherquality devices than the conventional one.

The entire disclosure of Japanese Patent Application No. 2003-422932filed on Dec. 19, 2003 including claims, specification, drawings, andabstract are incorporated herein by reference in its entirety.

As many apparently widely different embodiments of the present inventioncan be made without departing from the spirit and scope thereof, it isto be understood that the invention is not limited to the specificembodiments thereof except as defined in the claims.

INDUSTRIAL APPLICABILITY

The inventive exposure apparatus injects carbon dioxide into the fluidused for the immersion method, and restrains the static electricity thatwould otherwise generate on the substrate surface.

The inventive device manufacturing method can transfer an image of adevice pattern on the mask onto a device substrate with high precision,and manufactures highly integrated devices, which are hard tomanufacture in the prior art.

1. An exposure apparatus comprising: a projection optical system forprojecting a pattern of a mask onto a substrate; and a fluid supply unitfor supplying a fluid between said projection optical system and thesubstrate, said fluid supply unit including an injection unit forinjecting carbon dioxide into the fluid, wherein said fluid supply unitincludes a degassing unit for degassing the fluid, said degassing unitbeing located at an upstream side of the injection unit.
 2. An exposureapparatus according to claim 1, wherein said injection apparatusincludes a membrane module for injecting the carbon dioxide.
 3. Anexposure apparatus according to claim 1, wherein the injection unitinjects the carbon dioxide at a concentration of the carbon dioxide inthe fluid between 0.02 ppm and 750 ppm.
 4. An exposure apparatusaccording to claim 3, wherein the injection unit injects the carbondioxide at the concentration of the carbon dioxide in the fluid between0.06 ppm and 300 ppm.
 5. An exposure apparatus according to claim 1,wherein the fluid supply unit includes a resistivity meter for measuringa resistivity value of the fluid, and the injection unit injects thecarbon dioxide based on a measurement result of the resistivity meter.6. An exposure apparatus according to claim 1, wherein the injectionunit injects the carbon dioxide so that a resistivity value of the fluidis between 0.02 MΩ·cm and 10 MΩ·cm.
 7. An exposure apparatus accordingto claim 6, wherein the injection unit injects the carbon dioxide sothat the resistivity value of the fluid is between 0.04 MΩ·cm and 5MΩ·cm.
 8. A device manufacturing method comprising the steps of:exposing an object using an exposure apparatus according to claim 1 anddeveloping the exposed object.